Today in science class we did an experiment to test the reaction when IODINE was added to OIL. One of the steps was to put 10ml each of different oils into separate tubes. After that we heated...

Today in science class we did an experiment to test the reaction when IODINE was added to OIL. One of the steps was to put 10ml each of different oils into separate tubes. After that we heated water in a beaker on the hot plate and put the tubes in to heat the oil. Something strange happened when my group was left with the last tube.There was some water leaking onto the hot plate. We were very confused. The teacher came to our area and when she tried to removed the beaker from the hot plate we were even more confused.The beaker split in two halves, top and bottom. We could not remove the bottom of the beaker. It was too hot. Hot water spilled but we where far enough away that we weren't burned. We are still thinking about why the beaker broke. We think it's because the beaker was thick and the water could not transfer the heat. Do you have any ideas as to how this could have happened?

It sounds like the beaker may have had a crack in it before you began the procedure. It's important for safety reasons to always inspect glassware before you begin a lab exercise. A very small crack can cause glassware to break when heated. Another possible cause is a manufacturing defect in which an area of inconsistency in the glass created a stress point.

Ordinary glass expands when heated. Glass is a poor conductor of heat, so the outside usually expands faster than the inside and the resulting stress breaks the glass. Lab glassware is made of borosilicate glass, which doesn't expand much when heated. The addition of boron and silica to glass reduces the amount of expansion and makes the glass stronger and more heat resistant. Pyrex, which is used to make oven-safe baking dishes, is also used for beakers and other lab ware.

Even borosilicate glass has its limits. If a beaker boils dry its temperature will increase to beyond its limit and it will break. As long as water is present in the beaker the water will absorb the heat and keep the temperature of the glass nearly constant. Since your beaker contained water when it broke it was most likely not overheated. It's a characteristic of borosilicate glass that when it breaks it usually breaks along a line into two or just a few pieces rather than shattering, so what you observed wasn't unusual. Again, it's likely that the beaker started out with a crack that went unnoticed.